Is salt, just salt … or not?

Is salt, just salt … or not?

There are countless sayings about salt. People speak of the “salt of the earth” or of “salt as a symbol of friendship”. Above all, though, there is “white gold”, because salt has played a central role in the food industry for millennia – as a flavour enhancer and as a preservative. Not to forget animal salts, as well as the group of so-called industrial salts, de-icing salts and gritting salts used for winter road services. This last group alone accounts for almost two thirds of total demand.

In recent years I have visited various salt extraction sites, including Guérande (on the French Atlantic coast) and Aigues-Mortes on the Mediterranean, where certain algal growths often make some of the ponds glow in shades of red. I have also been to the German salt mines in Bad Reichenhall and Berchtesgaden, where visitors traditionally hurtle down long wooden slides and row across underground lakes. The greatest challenge, however, was a scientific tour into the prehistoric underworlds of Hallstatt (Upper Austria), where rock salt has demonstrably been mined in deep galleries for thousands of years. Salt is, without question, a piece of cultural history.

What types of salt are there, and how is “fleur de sel” formed?

When it comes to quality, is origin more important, or the way it is processed? Why do salts come not only in white, but in many colours – from pink to green to black? To explore these questions more closely, I joined a guided tour of the Swiss Salt Works in Basel.

Broadly speaking, there are three types of salt: vacuum salt (boiled salt), sea salt and rock salt. Each type is obtained using different methods. The salts produced by the Swiss Salt Works are cooking salts and vacuum salts. At the sites of Schweizerhalle, Riburg and Bex, they are produced by leaching rock-salt layers at depths of up to 400 metres using injected water. What nature provides is not a finished product, but raw brine: a solution which, besides ordinary table salt (NaCl), contains further dissolved accompanying elements. To turn this raw material into “white gold” for various applications, a purification process is essential after leaching.

The concentrated salt solution flows from the drilling fields via pipelines to the salt works, where it is first softened and then enters the crystallisation process in an evaporator plant. In the past, brine here too was boiled down by hand in large pans – following the same principle, though with a different starting material, as traditional sea-salt production.

The model was – and still is – the traditional harvesting of sea salt, as seen in the salt gardens of southern countries. In Europe these include, for example, flat coastal stretches in France (Brittany, the Camargue), Greece or Sicily. Sea salt crystallises through evaporation – in natural lagoons or in artificially constructed salt pans. It can be harvested by machine or by hand. When it is still laboriously skimmed from the surface with spades or rakes, it is known as fleur de sel. The large-grained salt crystals produced in this way are especially prized by (top) chefs. Because their grain is irregular they are crunchier, but they do not contain more, or higher-quality, nutrients or minerals. Fleur de sel contains slightly more sulphate (calcium sulphate), which creates a somewhat softer taste on the tongue. Specialists like to talk about “sensory notes” in such cases, as with the “Flor de Sal d’Es Trenc” from Mallorca.

The Basel salt works

It was the German Carl Christian Friedrich Glenck who discovered accessible salt deposits in 1837 and contributed his know-how to their extraction – laying the foundation for the first Swiss salt works. Before then, transporting salt imported from France took six to eight weeks, and after passing 70 customs stations along the route, its price had risen sixteen-fold. It was – and is – worth its weight in gold.

Depending on demand, the Swiss Salt Works produce between 400,000 and 500,000 tonnes per year. What is first explained using a model and a film can then be experienced live on the guided walk around the site: pipelines everywhere, conveyor belts, enormous boilers, tanks, huge centrifuges – plus filling and packaging lines and a vast, fully automated high-bay warehouse. And best of all: gigantic storage halls which, at full capacity, can hold around 120,000 tonnes of salt. More on that later.

Brine purification: from the “Schweizerhalle process” to a patented improvement

During the tour, purification takes on particular significance, because it is here that one sees how a traditional industry can be modernised. Since the beginning of the twentieth century, a reference process has been in use – the “Schweizerhalle process”, also known as the “lime-soda process”, filed as German patent DE 140605 on 3 April 1903 by Von Glenck Kornmann & CIE FA. The logic is simple and effective: unwanted substances are precipitated from the brine in two stages by adding quicklime, soda ash and carbon dioxide. Historically, this was a major leap in quality, because salt in earlier generations could – depending on composition – taste bitter or sour, and depending on origin might be slightly brownish or blackish. With Glenck and Kornmann’s innovation, the production of a pure, white salt became established – that very “white gold”.

Today, more than a century later, the same process step is once again under scrutiny. Martin Lauber, Head of Quality Control at the Swiss Salt Works and responsible for chemical, physical and microbiological analyses, sums it up with a smile: “Unfortunately, nature doesn’t give us JuraSel straight out of the ground.” What is extracted is raw brine – and to turn it into a product, a purification process is still required. Lauber explains that years of operation have shown the classic method can be refined further, making salt production more sustainable while also more economical.

Over the past three years, this improvement has been pursued with technical thoroughness – right up to computer simulation of brine purification, where ions, pH values and solubility shape daily work. The evaluation of the trials shows that, in two separate steps, a substantial portion of reagents can be saved: up to 20 per cent of quicklime and 40 per cent of soda ash used for brine purification. Initial adjustments have already been implemented; according to the company, this reduces the Swiss Salt Works’ indirect greenhouse-gas emissions by 6.6 per cent.

The word “indirect” is crucial here: it refers not only to emissions coming directly from an on-site chimney, but to those generated beyond the factory fence – for instance through purchased energy and the supply chain. In corporate carbon accounting, this includes emissions from bought electricity, steam, heat or cooling, as well as a significant share of the “embedded” emissions from goods and services procured from third parties.

In this case, the connection to CO₂ is particularly plausible because it concerns the materials used. Quicklime is produced from limestone; the industrial process releases CO₂ due to the chemical reaction itself and also requires high-temperature heat. Producing soda ash at industrial scale is also energy-intensive and therefore carries a corresponding emissions burden. A structural reduction in these inputs therefore cuts not only material consumption, but also upstream emissions where these reagents – and the energy needed for them – are produced.

Seen in this light, the figure of 6.6 per cent is not decorative, but the result of two reinforcing levers: fewer chemicals with a high footprint, and a more precise purification process which stabilises operational efficiency through better control of accompanying substances and deposits. The company also frames this innovation as another step towards its stated goal of “climate neutrality by 2040”.

To protect these innovations, the process has been filed internationally for patent protection. Specifically mentioned is the submission of an improvement to the Schweizerhalle process to the European Patent Office, with the expectation of worldwide approval in summer 2025.

Big salt bagger

The technology behind the perfect white

For leaching – or extracting – the brine, three pipes are lowered and fixed, in a way similar to fracking. The innermost pipe brings the saturated brine (around 33 per cent) up to the surface, the second supplies fresh water from above, and the outermost pipe carries nitrogen, which is pressed as a protective gas between the upper salt layer and the inflowing fresh water to prevent overly rapid leaching. A network of underground brine, water and protective-gas pipelines runs through the drilling fields for over 50 kilometres, ultimately converging at pumping stations. There, the brine becomes visible for the first time as a water-clear liquid and is pumped onwards into the reactor tanks of the purification plant.

In saturated raw brine, calcium and magnesium compounds are dissolved alongside sodium chloride. If these “secondary salts” were not removed, they would quickly encrust the heating chambers of the evaporators with a rock-hard deposit and drastically reduce the plant’s performance. For this reason, the accompanying substances are precipitated – the brine is softened. This happens in phases through the addition of substances such as quicklime, soda ash and carbon dioxide. The main residues are gypsum and lime, which are pressed and pumped into disused caverns.

Heating steam at 140 degrees brings the brine to the boil in the evaporator. The vapour produced is removed from the top. Salt continuously crystallises out and collects as a wet slurry at the bottom of the evaporator. At the Schweizerhalle works, six evaporators are currently in operation. They are over 20 metres high and each has a volume of 100 cubic metres. Compressors can recompress the so-called vapour steam and reuse it as heating steam. Thanks to this process, today one needs fifteen times less heating energy for salt production than decades ago.

After evaporation, the wet salt slurry is fed into centrifuges. At high rotational speed – like a washing machine’s spin cycle – water and salt are separated. Only two per cent remains as residual moisture in the salt. It now appears brilliantly white and pure, like freshly fallen powder snow. A fluidised-bed dryer provides the final drying step with streams of hot air, so that only a few millilitres of water remain per 100 kilograms of salt. To prevent deficiency conditions such as goitre, table salts may additionally be enriched with iodine and fluoride.

Aside: rock-salt mining and colour

Rock salt is salt mined dry, using underground mining methods, from salt-bearing rock layers. Salt mines are usually operated underground. In salt deserts such as the Salar de Atacama near Antofagasta in northern Chile, and at the edges of salt lakes, rock salt is also extracted above ground.

Ultimately, rock salt (also called halite) is nothing more than what ancient seas left behind millions of years ago as lagoons dried out. Through tectonic movement and eruptions, these salt layers were covered by other rocks and are therefore found underground or inside mountains. Famous – as already mentioned – is the historical Celtic rock-salt mining at Hallstatt around 1000 BC. Thanks to modern mining techniques, by far the largest share of global salt production today comes from underground or open-cast mining.

How varied salt appears in nature is already evident in its different colours, which are caused by mineral inclusions. Striking again and again is pink Himalayan salt, which contains higher levels of iron compounds, as well as black salt created by enrichment with volcanic ash or activated charcoal. Supposedly the most expensive salt in the world is also very dark and comes from an underground deposit in the Kalahari Desert in southern Africa.

What does salt mean for humans and animals?

A visit to any salt works is an intriguing experience and casts the “salt of life” in a new light. The average annual per-capita requirement for all salts is currently around six kilograms. A person’s daily requirement is 3 to 4 grams of salt, or sodium chloride (or, in some cases, potassium chloride). Otherwise health is at risk – especially the cardiovascular system, fluid balance and digestion. For livestock, such as a cow, the daily requirement is even 40 to 50 grams. Intake is provided either via salt licks or by mixing salt into feed.

In the right dosage, salts are genuine all-rounders: they also have antibacterial and anti-inflammatory effects. For this reason, bathing in highly saline water – such as the Dead Sea – is credited with therapeutic success for wounds and skin conditions.

What does the price say? Is only expensive salt edible as food or table salt, and is cheap de-icing salt unsuitable for consumption?

In the course of the production process, there is no difference to be detected. The guide smiles and agrees: in principle there is no difference; even de-icing salt could be used for cooking if it were stored cleanly and hygienically. This principle applies equally to all three types of salt. A remarkable insight – and certainly one of the best-kept value-for-money secrets. Given the low annual consumption per person, the kilo price of around one euro or Swiss franc hardly matters; indeed, salt is astonishingly inexpensive. Anything above that is luxury, or marketing. Stored dry, it also keeps indefinitely. In Germany and Austria, the salt trade is deregulated; however, there is a state monopoly on prospecting for and extracting rock salt, which lies with the state.

The highlight of the tour

The grand finale of the walk-through is the huge storage halls – or domes. Outside Basel there are two of them. The Riburg works alone produces around 50 tonnes of salt per hour, which is stored here on demand. These domes are considered the largest wooden cupolas in Europe. They are 31 metres high, have a diameter of 93 metres and a capacity of 80,000 tonnes of salt.

The moment the door opens, you feel as if you have stepped into a snow – or ski – paradise, were it not for a spotless excavator right in the middle. A 20-metre-high mountain of the finest white gleams towards the visitor. In fact, the salt’s texture is like fine washing powder or sand, and with suitably wide skis one could glide down it wonderfully, as in “sand-dune skiing”. But caution: at the end of production, an anti-caking agent is sprayed onto the dried salt. Climbing it could cause sinking and a real bog-like effect. Technically, however, this step is essential, because otherwise the salt would clump into a cement-like lump at the latest during packaging. So climbing is not allowed – only a little, a few steps. What you may do, and indeed are meant to do, is throw salt over your left shoulder with your right hand, following miners’ tradition – it is said to bring good luck. A fine custom, which I am happy to observe.